The useable radio spectrum is limited and traditionally the available spectrum has been licensed to particular users or groups of users by governmental agencies, such as the Federal Communications Commission in the United States. This licensing paradigm may be on the cusp of change. In the article “The End of Spectrum Scarcity” published by IEEE Spectrum, the authors note that while some available spectrum is congested, much of it is underutilized. They predict a future where spectrum is cooperatively shared and where smart antennas will adaptively lock onto a directional signal and when used in a transmission mode, operate directionally as opposed to omnidirectionally.
In terms of sharing spectrum, one way of doing so is by the use of spread spectrum technologies. Ultra-wideband (UWB) technology uses ultra wide bandwidths (for example, in excess of 500 MHz) to transmit information which in theory at least should not interfere with existing narrow band licensees (whose narrow band transmissions have bandwidths in the 0.5 to 15 KHz range).
Another spectrum sharing technique which is currently under discussion is cognitive radio which envisions using underutilized portions of the radio spectrum on an as needed basis. Cognitive radio can adapt to using different parts or portions of the radio spectrum when those parts or portions are not being actively used by another user.
Both UWB and cognitive radio have a need for widebanded communication equipment, with bandwidths significantly wider than found in most conventional radio equipment today. It is believed that future radio equipment will operate over much wider bandwidths than typical radio equipment does today.
It is well known that the performance of electrically-small antennas (ESAs) is limited when using traditional (i.e. passive) matching networks. Specifically, ESAs have high quality factor, leading to a tradeoff between bandwidth and efficiency. The most common definition of a ESA is an antenna whose maximum dimension (of an active element) is no more than ½π of a wavelength of the frequencies at which the antenna is expected to operate. So, for a dipole with a length of λ/2π, a loop with a diameter of λ/2π, or a patch with a diagonal dimension of λ/2π would be considered electrically small.
ESAs are very popular. They allow the antennas to be small. But due to their smallness, they can be very narrow banded.
The conventional way of dealing with an antenna which is used with a receiver and/or a transmitter with operates over a frequency band, and particularly where the antenna is mis-sized (electrically small) compared the frequency to be utilized, is to use an antenna matching network. Antenna matching networks operate ideally only at some particular frequency and therefore if the transmitter or receiver changes frequency, the mating network should normally be retuned to try to obtain an ideal match between the transmitter or receiver.
A passive adaptive antenna match is taught by U.S. Pat. No. 4,234,960. The antenna in U.S. Pat. No. 4,234,960 is resonated by a passive tuning circuit that is adjusted using a motor. A phase detector senses the presence of reactance and drives the motor until the reactance has been eliminated. This has two disadvantages: 1) the bandwidth is narrow due to the use of a passive tuning circuit, which necessitates the use of coarse (frequency sensing) and fine adjust, and 2) the motor driven tuning is slower than electronic tuning.
A “RF-MEMS based adaptive antenna matching module” taught by A. V. Bezooijen, et al., 2007 IEEE RFIC Symposium, resonates the antenna with a MEMS switched capacitor array. A phase detector senses the phase of the input impedance and steps the capacitance of the matching circuit either up or down by 1 increment depending on the sign of the phase. Disadvantages: 1) a positive capacitance does not resonate a monopole-type ESA 2) passive matching circuit results in narrow-band solution for ESA; and 3) digital tuning gives limited number of states.
Non-Foster matching networks overcome the limitations of passive circuits by using active circuits to synthesize negative capacitors and negative inductors in the antenna matching networks. When placed correctly, these circuits can directly subtract the from the antenna's reactance. For example, a 6″ monopole antenna has a reactance that may be approximated by a 3 pF capacitor at frequencies well below resonance. When combined with a −3.1 pF non-Foster capacitor, the net reactance is given by a 93 pF capacitor (using Eqn. (3) below), which is a 30 times improvement since the reactance is reduced by 30 times.
There are two related problems with this approach that need to be addressed before non-Foster matching is robust enough to be deployed in products: stability and accuracy. Negative capacitance is achieved using feedback circuits whose stability depends on both the internal circuit parameters and the load impedance; instability leads to either oscillation (i.e. emission of a periodic waveform from the circuit) or latchup. Unfortunately, the optimal impedance match typically occurs near the point where the stability margin goes to zero. Since non-Foster matching involves the subtraction of large reactances, high accuracy (tolerance˜1/Q) is needed to ensure both stability and optimal antenna efficiency. Consider the example just given, where the 6″ monopole antenna, which has a reactance that may be approximated by a 3 pF capacitor at frequencies well below resonance, is combined with a −3.1 pF non-Foster capacitor. The match is theoretically better with a −3.05 pF non-Foster capacitor, but if the net capacitance goes negative (see Eqn. (3)), then the match is unstable. There will probably always be manufacturing tolerances in making both antennas and circuits or devices, but as accuracy improves, the better the match network can be designed using a non-Foster negative impedance capacitor whose absolute value is even closer to the capacitance of the antenna. But accuracy and stability are related since the accuracy (or lack thereof) by which components can be manufactured will impact the likelihood of an unstable situation arising by reason of the combined antenna impedance and match network impedance being negative.
Component and manufacturing tolerances, as well as temperature and environmental loading effects, suggest that even a 10% error may be challenging to achieve using prior art non-Foster circuits.
Having a robust non-Foster automatic tuning circuit for coupling a transmitter and/or a receiver to an antenna, especially a ESA, would be useful for use in automobiles since it would allow the antenna design to be further reduced in size which is turn can lead to more aesthetic automobile designs and in vehicles generally (including automobiles, trucks, trains, planes, ship and boats) where a smaller antenna is likely to reduce drag and thereby increase efficiency. There are many more applications for this technology, such as the cognitive and UWB radios mentioned above.